Mating between the two lineages compromises fitness through the production of low-quality hybrid offspring. So the botanists tested the prediction that individuals sampled from sites occupied by both lineages should have flowers that promote autonomous self-pollen deposition and self-fertilization as a result of selection to reduce deleterious reproductive interference.

To test the prediction Jiménez-López and colleagues visited 25 populations of L. arvensis from France, Portugal, Spain, Switzerland, and Tunisia. “Populations were visited during the peak of flowering, and each was categorized as belonging to the blue lineage, the red lineage or was a mixed population (red- and blue-flowered plants in the same population),” write the authors. “We sampled ten populations from the blue lineage, six from the red lineage and nine mixed populations, with 475 blue- and 430 red-flowered plants sampled in total.”

The team found that lateral herkogamy was more effective than vertical herkogamy in limiting self-pollen deposition. In the case of vertical herkogamy, only approach herkogamy was effective. This is where the stigma is presented ahead of the anthers, so that insects approaching the plant deposit pollen from elsewhere before picking up fresh pollen from the anthers.

Lineages showed consistent differences in herkogamy traits. In general, angles were smaller for blue than red flowers in most populations, and blue flowers showed approach herkogamy, while red flowers showed predominantly reverse herkogamy. In sympatry, the red lineage showed a reduction of both herkogamy traits while for the blue lineage only lateral herkogamy was reduced.

“Our study has uncovered an unusual combination of dynamic lateral and vertical modes of herkogamy in an annual plant with two genetically divergent lineages with different floral colours and that experience different levels of attractiveness to pollinators,” say Jiménez-López and colleagues. “The conformation, extent and timing of herkogamy is partially consistent with expectations for populations that vary in their susceptibility to pollinator limitation. At the same time, the extent to which these patterns vary between pure and mixed populations is consistent with selection, on one of the two interacting lineages, with selection to avoid reproductive interference between them via increased self-fertilization.”